This application claims priority of European Patent Application No. 98301648.6, which was filed on Mar. 5, 1998.
The present invention relates generally to a system and method for reducing the peak-to-average power ratio in multiple access communication systems, and more particularly to direct sequence code division multiple access (DS-CDMA) systems.
Direct sequence code division multiple access (DS-CDMA), a form of spread spectrum communication, provides multiple users access to a common communication channel, such as a radio band or optical fibre. A DS-CDMA transmitter combines a user data stream with a digital sequence, known as a spreading code, yielding a combination bit stream, sometimes referred to as a spread signal. A spreading code generator in the DS-CDMA transmitter generates the spreading code elements, or chips, at a rate that is typically faster than the user data stream. The spreading codes are designed to appear random, although the spreading codes are duplicated at a DS-CDMA receiver for use in recovery of a user data stream.
The spread signals are summed and filtered before transmission. The summing converts the multiple parallel combination bit streams to a serial signal, and filtering limits the frequency band occupied by the signal. Additional modulation techniques, such as Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK), are used to modulate a carrier signal. If a transmitter uses BPSK, the summing operation provides a digital value representing the amplitude of the summed signal for each element of the spreading code. In QPSK, however, the summing operation yields a complex digital value having an in-phase and a quadrature component.
At the DS-CDMA receiver, a spreading code generator duplicates a user""s spreading code for mixing with the received signal and recovery of the particular user data stream. Since each user has a unique spreading code, signals spread with other user codes, or signals not spread at all, resemble noise and are not recovered by the receiver. The use of different spreading code sequences by each user allows users to operate independently of each other, although all data streams are transmitted in the same band.
A problem with DS-CDMA signals is connected with peaks at the output of the summation. The CDMA modulation makes the data streams look like independent random signals, so the mean square sum is proportional to N, where N is the number of data streams. Thus, the average power is proportional to N. However, there is a finite probability that in a given chip period all of the CDMA modulated data streams will have their maximum value simultaneously. Thus the peak amplitude will be proportional N and the peak power to N2. The peak to average power (PAP) ratio thus increases with the number of data streams.
It is a concern in DS-CDMA systems, therefore, to experience signals with high peak to average power (PAP) ratios. A high PAP ratio reduces the efficiency of a transmitter power amplifier. If the power amplifier is allowed to saturate during periods of peak power, the distortion will cause harmonics of the input signal to appear outside the desired frequency band. The power level of the unwanted harmonics will vary with the performance of the individual power amplifier units. Out of band harmonics interfere with communication on adjacent channels.
Also, successful recovery of transmitted data in a DS-CDMA receiver relies upon the linearity of previous stages in the CDMA process. Maintaining linearity in a transmitter""s power amplifier becomes progressively difficult as the number of users increases, due to the random nature of the signal generated by the summation of multiple spread signals. Due to the high PAP ratio, the power amplifier must generate sufficient mean power to maintain the signal level at the receiver but also remain linear during peak periods. Design and manufacture of such power amplifiers is costly and complex.
There is a certain tradeoff between distortion and power efficiency in a DS-CDMA transmitter. As the number of users increases, the PAP ratio also increases. The amount of tolerable distortion depends on spectral regulation and system requirements. Efficiency can be improved by allowing for more distortion, but with a corresponding degradation in the transmitted signal. Power amplifiers designed to accommodate high peaks decrease distortion, but such amplifiers are costly to design and manufacture. It is therefore, a concern to reduce PAP ratio in a deterministic manner in order to achieve an optimal tradeoff.
For these reasons, it is highly desirable to reduce the PAP for simple and complex signals in a Ds-CDMA transmitter without giving rise to excessive errors or spurious signal components.
Toshifumi, U.S. Pat. No. 5,751,705 discloses a CDMA base station transmitter that limits an amplitude of a multiplexed spread signal to not exceed a predetermined value. The limiting is done by converting a complex signal in in-phase/quadrature form into polar (phase/amplitude) form, limiting the amplitude, and converting the resulting signal back to in-phase/quadrature form. FIG. 1 shows one example of a transmitter where the amplitude is limited. The CDMA modulator 22 provides the signal to a comparator 26. The comparator 26 compares the signal to a predetermined value. If the signal exceeds the predetermined value the amplitude of the signal is limited in a limiter 28. Digital filter 30 band limits the signal.
The present invention provides a system and method to reduce peak to average power (PAP) ratio in spread spectrum transmitters by hard limiting the output of the CDMA modulator whilst it is still in the form of a digital signal and prior to any frequency band filtering. Digital values representing components of a complex signal are detected. The magnitude of the vector sum of the components of the complex signal is approximated and compared to a predetermined threshold magnitude. Where the approximated magnitude exceeds the threshold magnitude, each component of the vector sum is reduced by a scaling factor so that the magnitude of the complex signal approximately equals the threshold magnitude. Applying a stage of digital filtering substantially reduces out of band harmonics.
The present invention also teaches apparatus to reduce PAP ratio in multiple access transmitters. A detector receives at least one digital value for each element of a spreading code. A magnitude detector determines the approximate magnitude of the complex digital value. A comparator determines whether the detected magnitude exceeds a predetermined threshold. A limiter applied in the digital domain reduces the magnitude of the detected digital value to a predetermined value. Out of band harmonics are substantially reduced by a digital filter applied to the clipped signal.